1. Field of the Invention
The present invention relates to a magnet roller incorporated in an electrophotographic device using an electrophotographic process in an image forming apparatus such as a copying machine, laser printer, or facsimile receiving equipment or the like.
2. Description of the Related Art
A magnet roller incorporated in an electrophotographic device is used as a development roller which performs development by supplying toner to an electrostatic latent image carrier and visualizing an electrostatic latent image or a cleaning roller which removes residual toner on the electrostatic latent image carrier after the visualized toner image is transferred to a sheet of paper. For example, when a magnet roller is used as a development roller, as shown in FIG. 9, a magnet roller 31 is constructed so that a body part 33 thereof made of a magnet material is formed around a shaft part 32, and is incorporated in a hollow cylindrical sleeve 34 made of an aluminum alloy etc. In the outer peripheral face of the body part 33 of the magnet roller 31 are provided a plurality of magnetic poles in a magnetized manner. A magnetic pole having a highest surface magnetic flux density of these magnetic poles is called a main magnetic pole, and is often used as a development pole. Conventionally, there has mainly been used a magnet roller in which a main magnetic pole (development pole) consisting of one magnetic pole is provided in the body part, and the surface magnetic flux density curve (magnetic force distribution curve) thereof exhibits a single high peak, or a magnet roller in which two magnetic poles with the same polarity are provided as main magnetic poles (development poles), and the magnetic force distribution curve thereof exhibits two high peaks (W peak).
The body part of the conventional magnet roller is made up of a bond magnet that is formed by injection molding or extrusion molding a material produced by mixing a resin binder such as a thermoplastic resin with strontium-based or barium-based ferrite magnetic powder or rare-earth magnetic powder (typical examples are Ndxe2x80x94Fexe2x80x94B based magnetic powder and Smxe2x80x94Co based magnetic powder). The magnetic characteristics necessary for the magnet roller are provided by applying an external magnetic field to the body part of the magnet roller to magnetize it at the time of molding or after the molding.
However, the conventional magnet roller mainly has problems of the following items (1) to (4).
(1) The magnet roller using ferrite-based magnetic powder cannot meet the requirement for high magnetic force. For the magnet roller in which the main magnetic pole is formed by a single magnetic pole, the magnetic force of the magnet roller having, for example, an outside diameter of 13.6 mm is 850 G at the maximum, and, even if a strong magnetic field (about 30 KOe) is applied at the time of magnetization, it is difficult to obtain a high magnetic force of 850 G or higher because of magnetic saturation.
(2) In recent years, a magnet roller in which the main magnetic pole is formed by a plurality of magnetic poles has been developed. However, the magnetic force at the main magnetic pole thereof is 600 G or lower, and thus the magnetic pole cannot have a high magnetic force capable of sufficiently playing a role as a development pole. The reason for forming the main magnetic pole by a plurality of magnetic poles is that the range of chain phenomenon of developer in the circumferential direction is wide, which offers an advantage of increasing the development efficiency.
(3) On the other hand, the magnet roller using rare-earth magnetic powder is barely able to provide a low magnetic force (about 700 G) by the magnetization in a weak magnetic field because the coercive force of rare-earth magnetic powder is relatively high (intrinsic coercive force (iHc): 5 KOe or higher). Therefore, a strong magnetic field (about 20 to 30 KOe) must be applied to obtain a high magnetic force, so that a magnetizing apparatus must inevitably be large in size and require high power, which results in complicated magnetizing process and high cost.
(4) The conventional rare-earth magnetic powder has a low Curie point of about 330xc2x0 C., so that the use limit temperature thereof is restricted to a low temperature of about 130xc2x0 C. Also, the conventional rare-earth magnetic powder has poor corrosion resistance and oxidation resistance, so that rust etc. are formed to decrease the magnetic characteristics. In order to prevent rust etc. from being formed, a surface coating such as plating is needed, which brings about an increased cost.
The present invention has been achieved to solve the above problems, and accordingly an object thereof is to provide a magnet roller capable of obtaining a high magnetic force even if the magnet roller is magnetized in a relatively weak magnetic field and capable of being produced at a low cost. In particular, another object of the present invention is to make the magnetic force of a main magnetic pole formed by a single magnetic pole a high magnetic force of 850 G or higher even if the magnet roller is magnetized in a weak magnetic field of 15 KOe or lower and to make the magnetic force of a main magnetic pole formed by a plurality of poles at a practically sufficient level. At the same time, still another object of the present invention is to provide a magnet roller in which a surface coating such as plating is not needed, and the corrosion resistance and oxidation resistance are high.
To attain the above objects, the inventor paid attention to a xe2x80x9cnanocomposite magnetxe2x80x9d made up of a soft magnetic material having a low coercive force and a hard magnetic material, in which the crystal grain size of the soft magnetic material is on the order of nanometer, and carried out studies earnestly on the magnet of this type. As a result, the inventor found that the nanocomposite magnet is suitable as a magnet material for a magnet roller, and came up with the present invention.
Specifically, the present invention provides a magnet roller comprising a body part and a shaft part supporting both ends of the body part, in which a plurality of magnetic poles are provided in a magnetized manner in the outer peripheral face of the body part, wherein the whole or a part of the body part is made up of a rare-earth bond magnet made of rare-earth magnetic powder, having a composite phase of a hard magnetic phase and a soft magnetic phase both magnetically exchange-interacting with each other and having a coercive force (iHc) of 5 KOe or lower and a residual magnetic flux density of 5 KG or higher, and a resin binder. Therefore, there can be obtained a magnet roller having magnetic characteristics of a low coercive force (iHc) provided by the presence of soft magnetic phase and a high residual magnetic flux density (Br) provided by magnetic exchange-interaction.
Also, the rare-earth magnetic powder preferably consists of exchange spring magnetic powder. xe2x80x9cExchange spring magnetismxe2x80x9d is defined as a magnetic property that when a large amount of soft magnetic phase exists in a magnet, crystal grains of this soft magnetic phase and a hard magnetic phase are connected to each other by magnetic exchange-interaction, by which the magnetization of soft magnetic phase, which intrinsically has only a low coercive force and is easily reversed in a reverse magnetic field, becomes difficult to reverse even in the reverse magnetic field, and a mode looking as if both phases are connected to each other by a spring and thus a single phase consisting of hard magnetic phase only is exhibited (for example, see R. Coehoorn, K. H. J. Buschow et al.: J. de Phys., 49 (1988) C8-669).
The rare-earth magnetic powder using rare-earth elementxe2x80x94ironxe2x80x94boron compound phase as the hard magnetic phase and iron phase or ironxe2x80x94boron compound phase as the soft magnetic phase, or the rare-earth magnetic powder using rare-earth elementxe2x80x94ironxe2x80x94nitrogen compound phase as the hard magnetic phase and iron phase as the soft magnetic phase is suitable. Since the rare-earth magnetic powder of this kind contains a large amount of soft magnetic phase, the Curie point, which is an index of temperature dependence of residual magnetization, is mainly governed by the temperature dependency of soft magnetic phase. Therefore, the Curie point of the rare-earth magnetic powder takes a high value of about 400xc2x0 C. or higher, and the temperature dependency of residual magnetization becomes low, so that the use limit temperature can be made as high as 200xc2x0 C. or higher.
Also, it is preferable that 1 to 16 wt % of cobalt (Co) be added to the rare-earth magnetic powder. Thereby, a bond magnet manufactured of the rare-earth magnetic powder is caused to contain more Co than the conventional rare-earth Ndxe2x80x94Fexe2x80x94B based magnet consisting mainly of a hard magnetic phase, so that the corrosion resistance and oxidation resistance are increased, and also the occurrence of rust etc. can be prevented without a surface coating such as plating. Specifically, if the Co content is lower than 1 wt %, the oxidation resistance etc. of the bond magnet decrease so that rust etc. are liable to be formed. On the other hand, if the Co content exceeds 16 wt %, the coercive force of the bond magnet decreases, so that it is difficult to maintain the magnetic characteristics necessary for the magnet roller.
Also, when a main magnetic pole is formed by a plurality of magnetic poles, it is preferable that the polarities of the adjacent magnetic poles of the magnetic poles forming the main magnetic pole be made reverse to each other. By making the polarities of the adjacent magnetic poles reverse to each other, the reversion (rotation) of developer caused by a change of magnetic polarity in the development zone (a zone in which developer chains toward a photosensitive material on the main magnetic pole) is generated with a rotation of the magnet roller in use, which offers an advantage that the supply efficiency of developer to a photosensitive material can be increased.
In order to control the magnetic force distribution of magnet roller, it is preferable that a magnet piece made up of the rare-earth bond magnet be provided in a groove formed along the axis near the main magnetic pole in the outer peripheral face of the body part. This rare-earth bond magnet can be formed by a single or a plurality of magnet pieces. When the main magnetic pole is formed by a plurality of rare-earth bond magnet pieces, it is preferable that the polarities of the adjacent rare-earth bond magnet pieces be make reverse to each other.
Also, the magnet roller may be formed by bondedly providing a plurality of magnet pieces each consisting of the rare-earth bond magnet on the outer peripheral face of the shaft part. For example, the magnet roller may be formed by bonding magnet pieces having a C-shaped cross section, which consist of the conventional ferrite resin magnet or the like in the outer peripheral face of the shaft part, and by fitting a single or a plurality of rare-earth bond magnet pieces in a C-shaped opening portion (near the main magnetic pole) of the magnet piece.